Field of the Disclosure
Embodiments of the present disclosure relates to the art of a power transmission unit for delivering torque of a prime mover to drive wheels, and especially to a power transmission unit having a torque limiter for preventing the prime mover and the power transmission unit from being subjected to an overload.
Discussion of the Related Art
JP-A-2009-120043 describes a drive unit for a hybrid vehicle having an engine, a first motor and a second motor, and a single-pinion planetary gear unit for distributing torque of the first motor to the engine and to the drive wheels. According to the teachings of JP-A-2009-120043, in the planetary gear unit, a carrier is connected to the engine, a sun gear is connected to the first motor, and a ring gear is connected to an output member for delivering power to the drive wheels. In the drive unit taught by JP-A-2009-120043, an operating mode can be selected from an electric mode in which the vehicle is powered at least by one of the first motor and the second motor while stopping the engine, a regeneration mode in which at least one of the first motor and the second motor is operated as a generator, and an engine mode in which the vehicle is powered by the engine. In order to halt a rotation of the engine during propulsion under the electric mode, the drive unit taught by JP-A-2009-120043 is provided with a dog clutch serving as a locking device in a casing holding the motors and the planetary gear unit.
US2015/0040720 A1 describes a vehicular transmission including a transmission input shaft to which engine torque is input, and a torque limiter arranged between the transmission input shaft and an engine. According to the teachings of US2015/0040720 A1, the torque limiter is configured to enable torque transmission between the transmission input shaft and an engine output shaft, and to inhibit an input of excessive torque larger than predetermined torque between the transmission input shaft and the engine output shaft. The vehicular transmission taught by US2015/0040720 is also used in a hybrid vehicle having an engine and two motors, and further provided with a one-way clutch serving as a locking device to prohibit reverse rotation of the engine while allowing normal rotation of the engine. The one-way clutch is fixed to a flywheel connected to an output shaft of the engine between the engine and a transmission, and the torque limiter and a damper device are also fixed to the flywheel outside of a transmission casing.
According to the teachings of JP-A-2009-120043, a drag loss of the engine can be reduced by halting (i.e., locking) a rotation of the engine by the locking device during propulsion under the electric mode. In addition, during propelling the vehicle by both motors, an output torque of the first motor can be delivered efficiently to the output member through the planetary gear unit by halting a rotation of the carrier together with the engine. In the drive unit taught by JP-A-2009-120043, a powertrain between the engine and the drive wheels can be prevented from being subjected to an excessive torque by arranging the torque limiter taught by US2015/0040720 in the powertrain. However, if the locking device is actuated during propulsion under the electric mode, the torque limiter may not function in the powertrain. In this situation, if an excessive disturbance torque is applied from the drive wheel, gears and shafts of the powertrain between the drive wheels and the locking device may be subjected to an excessive load.